A plastic refractory is a moist, pliable mixture of aggregates and binders which when applied to a furnace wall or the like and fired in place forms a hard, monolithic, refractory lining for the substrate. Plastic refractory compositions are manufactured in granular form and in slab form. Both forms have been and still are placed by ramming the plastic masses onto the substrate to knead and knit them together and form a monolithic lining. Ramming is labor intensive and much care must be taken to avoid lamination of the plastic masses which would shorten the useful life of the refractory lining. Granular plastic refractories may also be formulated as gunning compositions.
The gunning of granular plastic refractories has the advantages of being potentially less labor intensive and of forming non-laminar linings. The potential for savings in labor costs is reduced in some circumstances by the large amount of refractory material that rebounds from the surface being covered. Also, the larger pieces of aggregate may be lost by rebounding while the smaller ones remain, resulting in the emplacement of a refractory lining widely different in composition than the starting material. In some cases the rebounded material can be recovered and reworked into a pliable mass but oftentimes the rebounded material is not usable because of contamination. In either circumstance, time is lost in recovering the rebounded material and in gunning for a longer period to build up the desired thickness. Moreover, the very nature of the plastic refractory makes it susceptible to compaction during storage and even during shipment. Thus, a plastic refractory which has been granulated for application by the gunning technique may not be in gunnable condition at the site because of compaction. Also, attempts to minimize the amount of rebound by increasing the amount of water in the refractory mixture have often had the undesirable effect of causing the mixture to cling to the internal surfaces of the delivery hoses and pumps of the gunning apparatus and eventually clogging the apparatus, thereby causing work stoppages.
A recent solution to the problem of maintaining the flowability of plastic gunning compositions during their application is disclosed in U.S. Pat. No. 4,952,534, incorporated herein by reference, which teaches that the flowability of such compositions may be improved by the addition of a hydrocolloid to the gunning mix resulting in smoother flow through the gunning apparatus and a decrease in rebounds. Nevertheless, while the addition of a hydrocolloid alone is usually sufficient under most conditions to insure acceptable gunning characteristics for most refractory gunning plastic compositions, it has now been found that in some situations certain compositions, even though containing a hydrocolloid, are more sensitive to manufacturing and use site variables resulting in poor and irregular flow characteristics of those compositions.
More particularly, it has been noted that in gunning operations, as the granulated plastic moves through the gun and hoses during installation, the effect of friction increases the temperature of the inner surfaces of the feedwheel, pockets, gun coupling and hoses. Gunning plastic compositions are composed of many ingredients of various particle sizes. The finer sized portion, generally composed of alumina, clay and bonding agent, make up a coating that surrounds the coarse aggregate. Due to their larger amount of surface area the finer sized materials of the mix are more reactive than the coarser grains. Further, the finer sized portion of the mix is also the first material to come into contact with the hot surfaces of the gun and hose. As more and more material is gunned, the finer portion can begin to harden on the interior metal parts of the gunning system. This in turn attracts more of the finer material creating a layered effect. As each layer forms coarser grains can become trapped in the coating which is formed forming a hardening, sticky blockage. Additionally, in so called fast-fire versions of these mixes, the fibers which are added to provide the fast-fire feature also become trapped in the coating and can form a layer comprised of an interlocking weave of fines, fibers and aggregate. Over time, these layers build up until excessive pressure is required to force the gunning plastic through the shrinking opening, ultimately resulting in blockage of the apparatus. In effect, it appears that the exposure of the plastic gunning composition to elevated temperatures during the gunning process causes a coating to accumulate on the interior surfaces of the gun and hoses resulting in blockage. Also, if the gunning apparatus operation is carried out at relatively high ambient temperatures the hardening of the layers occurs at an accelerated rate.